Soybean (Under development)

Soybean rust (Phakopsora pachyrhizi)

The most common symptoms are grey green, tan to dark brown or reddish brown lesions particularly on the undersides of the leaflets. These lesions are called uredia and they contain spores of the fungus. The spores from uredia are called urediniospores. Lesions tend to be angular, are restricted by leaf veins, and reach 2-5 mm in diameter. Lesions may also appear on petioles, pods, and stems. With time uredia turn black and at this stage they are called telia and the spores they contain are called teliaspores.

Lesions are frequently associated with leaf chlorosis, and high lesion densities result in premature defoliation and early maturity. During the early stages of development before the onset of sporulation, rust lesions may be confused with bacterial pustules (see bacterial pustule/bean blight). However, the symptoms of the two diseases can be differentiated by the presence of multiple uredia in the rust lesion and by the irregular cracks that usually appear in host tissue with a bacterial pustule lesion. Rust epidemics are most severe during long periods of leaf wetness when the mean daily temperature is less than 28 degC. Urediniospores are the primary means of disease spread.

Soybean is susceptible at any stage of development, but symptoms usually appear from the middle to late in the season because a prolonged wet, cool period is required for infection and sporulation. Spread of urediniospores is by windblown rain. The pathogen is not seed-borne in soybean. Rust can reduce yield by as much as 90%.

Soybean mosaic potyvirus (SMV)

Symptoms vary with host, virus strain, plant age at infection, and environment. Most infected cultivars are slightly stunted with fewer pods that are sometimes dwarfed and flattened, without hairs, and without seeds. Trifolilate leaves have a mosaic of light and dark green areas that may later become raised or blistered, particularly along the main veins. Primary leaves of some cultivars may show necrotic local lesions, which merge, into veinal necrosis followed by yellowing and leaf abscission.

Seeds from infected plants may be mottled brown or black, depending on hilum colour. Seeds may be smaller and germination reduced as compared to seed from non-infected plants. Mottling does not indicate that the virus is present in seeds as not all mottled seeds contain virus and not all seeds from virus-infected plants are mottled. SMV is sap and graft-transmissible. At least 32 aphid species, belonging to 15 different genera, transmit the SMV in a non-persistent manner. Virus isolates may show some vector specificity. Infected plants resulting from transmission through seed play an important role in SMV epidemiology. Such plants are sources for primary inoculum of SMV. In most cultivars, seed transmission is less than 5%, but no transmission occurs in some cultivars while others can have levels as high as 75%.

Bean yellow mosaic virus (BYMV)

Leaves of infected plants are not distorted as in SMV. The younger leaves show a yellow mottling scattered in random areas over the leaflets or sometimes an indefinite yellow band along major veins. Rusty necrotic spots develop in the yellowed areas as the leaves mature. Infected plants are not noticeably stunted. The disease is transmitted by aphids and infected seeds.

Soybean bacterial blight (Pseudomonas savastonoi pv. glycinea)

Blight lesions are most conspicuous on leaves but also occur on stems, petioles, and pods. Small, angular, translucent, water-soaked, yellow to light brown spots appear on leaves. The centres soon dry out, turn reddish brown to black, and are surrounded by a water-soaked margin bordered by a yellowish green halo. Young leaves are most susceptible. Infected young leaves are distorted, stunted, and chlorotic. The angular lesions enlarge in cool, rainy weather and merge to produce large, irregular dead areas. P. savastonoi pv. glycinea over seasons in surface crop residue and in seeds. Seeds can be infected through the pods during the growing season, or they may be invaded during harvesting. Primary infections on cotyledons often result in secondary lesions on seedlings. The bacterium is spread during windy rainstorms and during cultivation while the foliage is wet. It is seed-borne.

Wildfire disease of soybeans (Pseudomonas syringae pv. glycinea)

Symptoms consist of light-brown necrotic spots of variable size, surrounded by broad yellow halos on the leaves. In damp weather the spots enlarge forming large dead areas on the leaf. Wildfire disease is commonly associated with bacterial blight and bacterial pustule infections. The bacteria causing wildfire are seed-borne and also are carried over in crop debris.

Soybean downy mildew (Peronospora manshurica)

Early stages of the disease are characterised by indefinite yellowish-green areas on the upper surface of the leaves. As the disease progresses, those infected areas become greyish brown or dark brown and have yellowish-green margins. Severely infected leaves fall off prematurely. A greyish mould-like growth develops on the under surface of the spots when weather is wet and temperatures are cool. The disease also attacks the pods and infects the seeds. The disease is favoured by cool temperatures and wet conditions.

Pod and stem blight (Diaporthe phaseolorum pv. sojae)

It kills the plants in the later stages of crop development. Stems, petioles, pods, seed and less frequently leaf blades may be infected. It can be easily identified by the numerous, small, black fungal fruiting bodies (pycnidia) that appear on the stems and pods of infected plants. On the pods, the pycnidia are scattered while on the stems they are usually arranged in rows. The disease is favoured by wet weather. The disease is seed-borne and can over-season on diseased stems in the field.

Bacterial postule (Xanthomonas axonopodis pv. glycines)

The symptoms are much like those of bacterial blight. At first they appear as small, yellow-green spots with reddish-brown centres, more conspicuous on the upper surface of the leaf. A small, raised pustule usually develops at the centre of the lesion, especially on the lower leaf surface. This is the stage at which the disease is most readily distinguished from bacterial blight. The pustule and the absence of water soaking serve to distinguish bacterial pustule from bacterial blight. The latter shows water soaking at the centre or the margin of the dead area in the early stages of infection. In bacterial pustule, small infections may run together and cause large, irregular brown areas surrounded by a yellow margin. Parts of the brown dead areas may crack, giving the leaf a ragged appearance. The bacteria causing bacterial pustule over-season in diseased leaves and are seed-borne. Some strains infect common bean and cowpea.

White mould (Sclerotinia sclerotiorum)

It is characterised by a rot at the base of the plant stem. The rot is covered by a cottony, fungal (mycelial) growth on which black irregularly shaped fungal bodies (sclerotia) are produced. Size of sclerotia varies from 2 to 22 mm in diameter. The disease infects the stem and pods. Large, black, round to irregularly-shaped sclerotia of varying size form on stems, which are partially covered with dense white mycelium. Sclerotia are also formed in the stem pith and are conspicuous when the stem is opened. Seeds may become infected within diseased pods. If infected early, seeds are flattened and shrivelled and sometimes replaced by black sclerotia. Attacked plants die prematurely, sometimes before seeds have formed. Infection occurs at flowering. Moderate air temperatures and frequent rains just prior to flowering to the pod development stage of growth favour the disease.

Damping-off diseases and Anthracnose (Colletotrichum truncatum and Glomerella glycines)

They are caused by an array of fungi including Colletotrichum truncatum and Glomerella glycines (cause of anthracnose). The diseases cause rotting of seeds before emergence from the soil or death of seedlings after emergence.

The two species of fungi produce similar symptoms on the petioles, stems and pods particularly when plants are nearing maturity. Diseased areas have dark sunken lesions. Under moist weather the lesions become covered with a pink spore mass. The disease is seed-borne. When infected soybean seeds are planted, many of the seeds rot in the soil. Those that emerge from the soil often have brown, sunken cankers on the cotyledons (seedling leaves). The fungi may grow from them into the young stems. The damping-off seedling losses are probably more serious phases of soybean anthracnose than symptoms on older plants. The disease is favoured by cool wet weather.

Purple seed stain (Cercospora kikuchii)

Symptoms of purple stain are most evident on the seeds, but the fungus also attacks leaves, stems and pods. On the seeds, discolouration varies from pink or light purple to dark purple and ranges from a small spot to the entire seed coat. Cracks often occur on the discoloured areas giving the seed coat a rough, dull appearance. When infected seeds are planted, the fungus grows from the seed coats into the cotyledons (seedling leaves) and into the hypocotyls (seedling stem). The fungus produces spores abundantly on infected seedlings and these spores serve as source of infection to leaves, stems and pods. The spores are spread by wind and rain splash.